(1) Field of the Invention
The present invention relates to an antenna system and particularly, but not exclusively, to a phased array antenna system having a plurality of antenna elements arranged in at least two sub-arrays. The antenna system is suitable for use in many telecommunications systems but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, the antenna system of the present invention may be used with third generation (3G) mobile telephone networks and the Universal Mobile Telephone System (UMTS). The invention also relates to a phase compensation apparatus for use in an antenna system.
(2) Description of the Art
Operators of cellular mobile radio networks generally employ their own base-stations, each of which includes one or more antennas. In a cellular mobile radio network, the antennas define the desired coverage area which is generally divided into a number of overlapping cells, each associated with a respective antenna and base station. Each cell contains a fixed-location base station which maintains radio communication with all of the mobile radios in that cell. The base stations themselves are interconnected by other means of communication, usually fixed land-lines arranged in a grid or meshed structure, allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network.
The antennas used in such networks are often composite devices known as phased array antennas which comprise a plurality (usually eight or more), or array, of individual antenna elements or dipoles. The direction of maximum sensitivity of the antenna, i.e. the direction of the main radiation beam or “boresight” of the antenna pattern, can be altered by adjusting the phase relationship between the signals fed to the antenna elements. This has the effect of allowing the beam to be steered to modify the coverage area of the antenna.
In particular, operators of phased array antennas in cellular mobile radio networks have a requirement to adjust the vertical radiation pattern (VRP), so as to alter the vertical angle of the main beam, also known as the “tilt”, since this has a significant effect on the coverage area of the antenna. Adjustment of the coverage area may be required, for example, owing to changes in the network structure or the addition or removal of other base stations or antennas in the cell.
The adjustment of the angle of tilt of an antenna is known and is conventionally achieved by mechanical means, electrical means, or both. When the angle of tilt of the antenna is adjusted mechanically, for example by mechanically moving the antenna elements themselves or by mechanically moving the housing (or “radome”) for the elements, this is known as adjusting the angle of “mechanical tilt”. When the angle of tilt of the antenna is adjusted electrically, for example by changing the time delay or phase of signals fed to each element (or group of elements) in the array without physically moving either the housing for the elements, the antenna elements themselves or any other part of the antenna radome, this is commonly referred to as adjusting the angle of “electrical tilt”.
The effect of adjusting either the angle of mechanical tilt or the angle of electrical tilt is to reposition the boresight so that it points either above or below the boresight established by conventional mechanical or electrical tilt mechanisms, and hence increases or decreases the coverage area of the antenna.
Heretofore, adjustment of the mechanical or electrical tilt of a cellular radio antenna has been possible only by manually adjusting the angle of tilt at the antenna itself, for example by physically moving the antenna housing or radome in the case of mechanical tilt adjustment or by adjusting mechanical devices for applying variable amounts of delay to the elements in the antenna in the case of electrical tilt adjustment.
One disadvantage of employing such mechanical or electrical tilt adjustment methods is that the methods are difficult and time consuming to perform. Additionally, such adjustment methods result in the direction of the boresight being fixed at the adjusted angle of tilt for all transmitted or received signals, until such time as the angle of tilt is adjusted again. Consequently, the antenna cannot be shared by more than one operator unless the tilt required by each operator is identical. In practice this is rare and, in general, operators require an individual angle of tilt in order to optimise the cell coverage of the antenna for their particular deployment of base stations.
In any event, although the sharing of base stations, antennas and facilities is desirable, there are problems involved in doing so. In the United Kingdom, respective transmit/receive frequency bands are allocated to five 3G operators for transmission between mobile radios and base stations. The five transmit bands, or operator frequency bands, are contiguous, as are the five receive bands, i.e. there are no gaps between adjacent frequency bands. Consequently, unless complex and accurate filtering of signals is employed by the transmitting and receiving apparatus associated with the antenna, the resulting overlapping and interference of signals has a deleterious effect on system performance.
One known base station architecture provides separate transmit and receive antennas whilst another common system employs a duplexer to allow a single antenna to be used for both transmission and reception. These arrangements are adequate when only one operator is required to use the base station and antenna but when more than one operator wishes to use the system there are difficulties.
One known solution for base station sharing is for each operator to use a different antenna. In practice, this is achieved by the use of a shared antenna mast supporting a number of antennas, one for each operator. However, in order to avoid mutual interference, the antennas require adequate separation and the height of the mast may need to be increased, or a stronger structure may need to be used, to enable the mast to withstand high winds. This increases the weight of the mast which, in turn, increases mast cost. Furthermore, sites which can accommodate a larger mast are difficult to obtain and planning permission or zoning problems may be encountered. Larger masts are also environmentally obtrusive and unsightly.
As a consequence, many operators of mobile cellular radio networks employ their own base stations with their associated masts and antennas. There is little sharing of sites and any sharing that does occur is limited to the sharing of the mast only and not the antennas. The introduction of the so-called third generation (3G) mobile radio system will demand an increased number of base station sites. Thus, there are likely to be difficulties in acquiring the necessary real estate, and site sharing will become an increasingly attractive option.